Molecular genetic, biochemical, and physiological methods will be used to study obligate symbioses between marines invertebrates (corals, anemones, giant clams) and dinoflagellates (zooxanthellae). Current work, which is examining how different species of hosts and symbionts associate in nature, will be extended during the first year of this project. The rsults will be applied to an experimental analysis of symbiont gene expression-- the larger component of this proposal. Gene expression studies will examine ribulose- 1,5 bisphosphate carboxylase/oxygenase (Rubisco), which catalyzes the fixation of carbon by life on earth. Symbiotic (and probably many other) dinoflagellates possess a formerly unprecedented Rubisco encoded by a diverse, nuclear multigene family (all other eukaryotic Rubiscos are of a different type, and encoded by single chloroplast genes). In theory, this remarkable situation should generate Rubiscos with novel properties. It must also be a significant factor in the physiology of animal-algal symbioses, which operate within a fixed carbon-based economy. This project will evaluate that theory from empirical data. It is postulated that Rubisco functional diversity, which is known among other organims, and additional (combinatorial) diversity, exists within a dinoflagellate and is manifested by the differential and specific experssion of Rubisco loci. Rubisco isoforms will be assyed in symbionts challenged with environmental variables to which they, or Rubisco in other systems, respond. Rejecting the null hypothesis ("There is no difference in Rubisco expression, among isoforms, among treatments") will define the biological significance of the multigene family. In parallel work, Rubisco genes will be characterized by DNA sequencing, using a model system. Together, these efforts will provide the understanding tht is needed to initiate specific, detailed investigations of this remarkable enzyme.